The present invention relates to methods of treating cancer using a combination of an antineoplastic agent and a compound which is an inhibitor of a prenyl-protein transferase.
Chemotherapy, the systematic administration of antineoplastic agents that travel throughout the body via the blood circulatory system, along with and often in conjunction with surgery and radiation treatment, has for years been widely utilized in the treatment of a wide variety of cancers. Unfortunately, the available chemotherapeutic drugs often fail patients because they kill many healthy cells and thus bring on serious side effects that limit the doses physicians can administer.
Prenylation of proteins by intermediates of the isoprenoid biosynthetic pathway represents a class of post-translational modification (Glomset, J. A., Gelb, M. H., and Farnsworth, C. C. (1990). Trends Biochem. Sci. 15, 139-142; Maltese, W. A. (1990). FASEB J. 4, 3319-3328). This modification typically is required for the membrane localization and function of these proteins. Prenylated proteins share characteristic C-terminal sequences including CaaX (C, Cys; a, usually aliphatic amino acid; X, another amino acid), XXCC, or XCXC. Three post-translational processing steps have been described for proteins having a C-terminal CaaX sequence: addition of either a 15 carbon (farnesyl) or 20 carbon (geranylgeranyl) isoprenoid to the Cys residue, proteolytic cleavage of the last 3 amino acids, and methylation of the new C-terminal carboxylate (Cox, A. D. and Der, C. J. (1992a). Critical Rev. Oncogenesis 3:365-400; Newman, C. M. H. and Magee, A. I. (1993). Biochim. Biophys. Acta 1155:79-96). Some proteins may also have a fourth modification: palmitoylation of one or two Cys residues N-terminal to the farnesylated Cys. While some mammalian cell proteins terminating in XCXC are carboxymethylated, it is not clear whether carboxy methylation follows prenylation of proteins terminating with a XXCC motif (Clarke, S. (1992). Annu. Rev. Biochem. 61, 355-386). For all of the prenylated proteins, addition of the isoprenoid is the first step and is required for the subsequent steps (Cox, A. D. and Der, C. J. (1992a). Critical Rev. Oncogenesis 3:365-400; Cox, A. D. and Der, C. J. (1992b) Current Opinion Cell Biol. 4:1008-1016).
Three enzymes have been described that catalyze protein prenylation: farnesyl-protein transferase (FPTase), geranylgeranyl-protein transferase type I (GGPTase-I), and geranylgeranyl-protein transferase type-II (GGPTase-II, also called Rab GGPTase). These enzymes are found in both yeast and mammalian cells (Clarke, 1992; Schafer, W. R. and Rine, J. (1992) Annu. Rev. Genet. 30:209-237). Each of these enzymes selectively uses farnesyl diphosphate (FPP) or geranyl-geranyl diphosphate as the isoprenoid donor and selectively recognizes the protein substrate. FPTase farnesylates CaaX-containing proteins that end with Ser, Met, Cys, Gln or Ala. For FPTase, CaaX tetrapeptides comprise the minimum region required for interaction of the protein substrate with the enzyme. The enzymological characterization of these three enzymes has demonstrated that it is possible to selectively inhibit one with little inhibitory effect on the others (Moores, S. L., Schaber, M. D., Mosser, S. D., Rands, E., O'Hara, M. B., Garsky, V. M., Marshall, M. S., Pompliano, D. L., and Gibbs, J. B., J. Biol. Chem., 266:17438 (1991), U.S. Pat. No. 5,470,832).
The prenylation reactions have been shown genetically to be essential for the function of a variety of proteins (Clarke, 1992; Cox and Der, 1992a; Gibbs, J. B. (1991). Cell 65: 1-; Newman and Magee, 1993; Schafer and Rine, 1992). This requirement often is demonstrated by mutating the CaaX Cys acceptors so that the proteins can no longer be prenylated. The resulting proteins are devoid of their central biological activity. These studies provide a genetic “proof of principle” indicating that inhibitors of prenylation can alter the physiological responses regulated by prenylated proteins.
The Ras protein is part of a signalling pathway that links cell surface growth factor receptors to nuclear signals initiating cellular proliferation. Biological and biochemical studies of Ras action indicate that Ras functions like a G-regulatory protein. In the inactive state, Ras is bound to GDP. Upon growth factor receptor activation, Ras is induced to exchange GDP for GTP and undergoes a conformational change. The GTP-bound form of Ras propagates the growth stimulatory signal until the signal is terminated by the intrinsic. GTPase activity of Ras, which returns the protein to its inactive GDP bound form (D. R. Lowy and D. M. Willumsen, Ann. Rev. Biochem. 62:851-891 (1993)). Activation of Ras leads to activation of multiple intracellular signal transduction pathways, including the MAP Kinase pathway and the Rho/Rac pathway (Joneson et al., Science 271:810-812).
Mutated ras genes are found in many human cancers, including colorectal carcinoma, exocrine pancreatic carcinoma, and myeloid leukemias. The protein products of these genes are defective in their GTPase activity and constitutively transmit a growth stimulatory signal.
The Ras protein is one of several proteins that are known to undergo post-translational prenylation. Farnesyl-protein transferase utilizes farnesyl pyrophosphate to covalently modify the Cys thiol group of the Ras CAAX box with a farnesyl group (Reiss et al., Cell, 62:81-88 (1990); Schaber et al., J. Biol. Chem., 265:14701-14704 (1990); Schafer et al., Science, 249:1133-1139 (1990); Manne et al., Proc. Natl. Acad. Sci USA, 87:7541-7545 (1990)).
Ras must be localized to the plasma membrane for both normal and oncogenic functions. At least 3 post-translational modifications are involved with Ras membrane localization, and all 3 modifications occur at the C-terminus of Ras. The Ras C-terminus contains a sequence motif termed a “CAAX” or “Cys-Aaa1-Aaa2-Xaa” box (Cys is cysteine, Aaa is an aliphatic amino acid, the Xaa is any amino acid) (Willumsen et al., Nature 310:583-586 (1984)). Depending on the specific sequence, this motif serves as a signal sequence for the enzymes farnesyl-protein transferase or geranylgeranyl-protein transferase, which catalyze the alkylation of the cysteine residue of the CAAX motif with a C15 or C20 isoprenoid, respectively. (S. Clarke., Ann. Rev. Biochem. 61:355-386 (1992); W. R. Schafer and J. Rine, Ann. Rev. Genetics 30:209-237 (1992)).
Other farnesylated proteins include the Ras-related GTP-binding proteins such as RhoB, fungal mating factors, the nuclear lamins, and the gamma subunit of transducin. James, et al., J. Biol. Chem. 269, 14182 (1994) have identified a peroxisome associated protein Pxf which is also farnesylated. James, et al., have also suggested that there are farnesylated proteins of unknown structure and function in addition to those listed above.
Inhibitors of farnesyl-protein transferase have been described in two general classes. The first class includes analogs of FPP, while the second is related to protein substrates (e.g., Ras) for the enzyme. The peptide derived inhibitors that have been described are generally cysteine containing molecules that are related to the CAAX motif that is the signal for protein prenylation. (Schaber et al., ibid; Reiss et. al., ibid; Reiss et al., PNAS, 88:732-736 (1991)). Such inhibitors may inhibit protein prenylation while serving as alternate substrates for the farnesyl-protein transferase enzyme, or may be purely competitive inhibitors (U.S. Pat. No. 5,141,851, University of Texas; N. E. Kohl et al., Science, 260:1934-1937 (1993); Graham, et al., J. Med. Chem., 37, 725 (1994)).
Mammalian cells express four types of Ras proteins (H—, N-, K4A-, and K4B-Ras) among which K-Ras4B is the most frequently mutated form of Ras in human cancers. Inhibition of farnesyl-protein transferase has been shown to block the growth of H-ras-transformed cells in soft agar and to modify other aspects of their transformed phenotype. It has also been demonstrated that certain inhibitors of farnesyl-protein transferase selectively block the processing of the H-Ras oncoprotein intracellularly (N. E. Kohl et al., Science, 260:1934-1937 (1993) and G. L. James et al., Science, 260:1937-1942 (1993). Recently, it has been shown that an inhibitor of farnesyl-protein transferase blocks the growth of H-ras-dependent tumors in nude mice (N. E. Kohl et al., Proc. Natl. Acad. Sci U.S.A., 91:9141-9145 (1994) and induces regression of mammary and salivary carcinomas in H-ras transgenic mice (N. E. Kohl et al., Nature Medicine, 1:792-797 (1995).
Indirect inhibition of farnesyl-protein transferase in vivo has been demonstrated with lovastatin (Merck & Co., Rahway, N.J.) and compactin (Hancock et al., ibid; Casey et al., ibid; Schafer et al., Science 245:379 (1989)). These drugs inhibit HMG-CoA reductase, the rate limiting enzyme for the production of polyisoprenoids including farnesyl pyrophosphate. Inhibition of farnesyl pyrophosphate biosynthesis by inhibiting HMG-CoA reductase blocks Ras membrane localization in cultured cells. Because prenyl pyrophosphates are intermediates in many biosynthetic processes, direct inhibition of a prenyl-protein transferase would be more specific and attended by fewer side effects than would occur with the required dose of a general inhibitor of isoprene biosynthesis.
It is the object of the instant invention to provide a composition that comprises an antineoplastic agent and a prenyl-protein transferase inhibitor whose therapeutic effect in combination may allow use of tie antineoplastic agent at a dose which is lower than the dose of the antineoplastic agent if it was used alone and may therefore ameliorate some of the unwanted side effects normally associated with traditional chemotherapy.
A pharmaceutically effective combination of an antineoplastic agent and a prenyl-protein transferase inhibitor are used in the present invention to treat cancer, such as in tumor cells that are less susceptable to treatment by antagonist of the antineoplastic agent or prenyl-protein transferase inhibitor when administered alone.